Method of controlling secondary condenser duty

ABSTRACT

A portion of the overhead vapor from a distillation column is used to heat a process stream. The temperature and flow rate of the stream out of a heat exchanger are controlled so that the vapor is condensed.

BACKGROUND OF THE INVENTION

Dehydrogenation process units often include fractionators and otherprocess equipment. A feed rich in the desired alkane can be separated ina distillation column into an overhead containing the desired alkane anda bottoms stream containing heavier hydrocarbons. The overhead vapor iscondensed and sent to the dehydrogenation unit for conversion to thealkene.

For example, in a propylene plant, a feed 10 rich in propane is sent toa depropanizer 15, as shown in FIG. 1. The feed 10 is separated into abottoms stream 20 containing butanes and heavier hydrocarbons and anoverhead stream 25 containing propane. The overhead stream 25 is sent toa condenser 30 where the overhead stream 25 is condensed into a liquidpropane stream 35 which is sent to a receiver 40.

A small amount of the overheard vapor stream 25, hot vapor bypass stream27, is sent directly to the receiver 40 where it is condensed on thevessel wall and on the subcooled liquid surface in the receiver 40.

The process typically utilizes a single condenser 30, which ispositioned at or below the level of the receiver 40 The condenser 30typically uses water for cooling. The flow rate to the receiver 40 iscontrolled by a pressure controller 45 which measures the pressure ofthe overhead stream 25. The liquid propane stream 50 from the receiver40 is sent to the dehydrogenation reactor (not shown).

A portion 55 of the liquid propane stream 50 from the receiver 40 can bereturned to the depropanizer 15.

The feed coming to the depropanizer is typically at a temperature ofabout −35° C. However, the temperature needs to be raised to at leastabout 0° C. so that the temperature is appropriate for the feed guardbeds for the depropanizer. This is typically done using an availableprocess stream, such as the overhead vapor from a deethanizer column.

SUMMARY OF THE INVENTION

One aspect of the invention is a process for condensing an overheadvapor stream from a distillation column. In one embodiment, the processincludes dividing the overhead stream into two portions. The firstportion of the overhead stream is introduced into a first heat exchangerto exchange heat with a process stream thereby increasing a temperatureof the process stream and reducing the temperature of the first portionof the overhead stream to condense the first portion of the overheadstream. The flow rate of the first portion of the condensed overheadstream from the first heat exchanger is controlled. The first portion ofthe condensed overhead stream is introduced into a receiver. The secondportion of the overhead stream is introduced into a second heatexchanger to reduce a temperature of the second portion of the overheadstream to condense the second portion of the overhead stream. The secondportion of the condensed overhead stream is introduced into the receiverand combined with the first portion of the condensed overhead stream toform a condensed liquid.

Another aspect of the invention is a separation process. In oneembodiment, the process includes separating a product stream in adistillation column into a liquid bottoms stream and an overhead vaporstream. The overhead stream is divided into two portions. The firstportion of the overhead stream is introduced into a first heat exchangerto exchange heat with a process stream thereby increasing a temperatureof the process stream and reducing the temperature of the first portionof the overhead stream to condense the first portion of the overheadstream. The flow rate of the first portion of the condensed overheadstream from the first heat exchanger is controlled. The first portion ofthe condensed overhead stream is introduced into a receiver. The secondportion of the overhead stream is introduced into a second heatexchanger to reduce a temperature of the second portion of the overheadstream to condense the second portion of the overhead stream. The secondportion of the condensed overhead stream into the receiver is introducedand combined with the first portion of the condensed overhead stream toform a condensed liquid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of one embodiment of a prior art depropanizer.

FIG. 2 is an illustration of one embodiment of the process of thepresent invention.

FIG. 3 is an illustration of another embodiment of the process of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

A portion of the overhead vapor from a distillation column can be usedto heat a process stream while condensing the portion of the overheadvapor stream to a liquid. The overhead vapor stream from thedistillation column is split into two portions. The first portion flowsto a first heat exchanger positioned above the receiver for thecondensed vapor, while the second portion flows to the heat exchanger ofthe prior art system, which is positioned at the level of or lower thanthe receiver.

The first portion flowing to the first heat exchanger should be no morethat about 80% of the total overhead vapor. The remaining portion of theoverhead vapor, which is at least about 20% of the total overhead vapor,goes to the second heat exchanger. Putting the two heat exchangers inparallel allows for a lower pressure differential between the pressureat the top of the column and the pressure of the receiver.

The additional heat exchanger provides the energy needed to heat theprocess stream. In addition, the presence of the additional heatexchanger reduces the amount of cooling water used in the other heatexchanger.

The elevation of the first heat exchanger positioned above the receiverensures that there is sufficient pressure drop for the control valve inthe rundown line between the first heat exchanger and the receiver. Thepressure drop is typically at least about 10 kPa (0.1 bar).

There is a temperature controller in the rundown line which controls avalve in the line. Desirably, the set point of the temperaturecontroller is set to ensure total condensation of the portion of theoverhead vapor stream used to heat the process stream. It can be set fora small amount of subcooling, if desired. The temperature is typicallyat least about 1° C. less than the bubble point of the overhead vaporstream. It is desirable to minimize subcooling in order to maximizeduty.

The temperature of the liquid at the outlet of the elevated heatexchanger is measured, and the flow rate of the liquid from the heatexchanger is controlled by the valve so that all of the overhead vaporstream into the heat exchanger is condensed. The system is designed toobtain the maximum duty while still ensuring that total condensation ofthe overhead vapor occurs. The normal column pressure control system canfunction in its usual manner without hydraulic or other controlproblems.

The second portion of the overhead vapor stream is cooled in a secondheat exchanger with cooling water, as in the prior art. The cooledliquid from the second heat exchanger is also sent to the receiver. Theduty of the second heat exchanger is adjusted to pressure control thedistillation column. If the required duty is too small control of thecolumn pressure will be difficult.

The second heat exchanger is typically at the level of the receiver orlower. The higher the second heat exchanger is, the lower the pressuredrop will be across the hot vapor bypass valve. Positioning the secondheat exchanger lower than the receiver ensure a pressure drop across thehot bypass valve.

The hot vapor by-pass flow is adjusted to regulate the liquidaccumulation in the second heat exchanger. Liquid retention reduces theheat transfer coefficient and the temperature difference.

The process can be used to heat any process stream which needs to itstemperature raised, such as for example, preheating boiler feed water,and the like. In one embodiment, the distillation column overhead isused to heat the feed to the distillation column.

The process can be used with any distillation column[s] in whichcondensation of the overhead vapor stream is to occur (desirably totalcondensation), such as for example, depropanizers, naptha splitters,xylene splitters, and the like. Desirably, the overhead vapor stream iscompletely condensed. The presence of only liquid in the run down linefrom the first heat exchanger makes the pressure values easier todetermine, simplifying selection of appropriate control valves. However,in some situations, partial condensation may be acceptable.

The overhead vapor stream of two or more distillation columns can becombined, if desired.

FIG. 2 illustrates one embodiment of the present invention. The feed 110to the distillation column 115 is separated into bottoms stream 120 andoverhead vapor stream 125. The overhead vapor stream 125 is divided intotwo portions 155 and 160. The second portion of the overhead vaporstream 160 is sent to heat exchanger 130 where it is cooled with coolingwater. The condensed liquid 135 is sent to receiver 140. Hot vaporbypass 127 goes directly to the receiver 140 where it is condensed onthe vessel wall and the subcooled liquid surface.

The first portion of the overhead vapor stream 155 flows to a heatexchanger 165. The first portion of the overhead vapor stream 155 iscooled and condensed through heat exchange with process stream 180. Thepreheated process stream 190 is then sent to another process unit (notshown).

The cooled liquid 170 is sent to the receiver 140. There is atemperature controller 175 between the heat exchanger 165 and thereceiver 140. The temperature of the condensed vapor is measured, andthe flow rate to the receiver is controlled to ensure that all of thevapor has been condensed in the heat exchanger 165. The condensed liquid150 from the receiver 140 is sent for processing in a dehydrogenationunit (not shown), and a portion 155 can be returned to the distillationcolumn 115.

Another embodiment is illustrated in FIG. 3. In this embodiment, theprocess stream to be heated is the feed stream to the distillationcolumn. The feed 210 to the distillation column 215 is separated intobottoms stream 220 and overhead vapor stream 225. The overhead vaporstream 225 is divided into two portions 255 and 260. The second portionof the overhead vapor stream 260 is sent to heat exchanger 230 where itis cooled with cooling water. The condensed liquid 235 is sent toreceiver 240. Hot bypass vapor 227 goes directly to the receiver 240where it is condensed on the vessel walls and on the surface of thesubcooled liquid.

The first portion of the overhead vapor stream 255 flows to a heatexchanger 265. The first portion of the overhead vapor stream 255 iscooled and condensed through heat exchange with the feed 205. Thepreheated feed 210 is then sent to the distillation column 215. Thepreheated feed 210 can be sent to various other pretreatment units, suchas guard beds to remove impurities (not shown), before being introducedinto distillation column 215.

The cooled liquid 270 is sent to the receiver 240. There is atemperature controller 275 between the heat exchanger 265 and thereceiver 240. The temperature of the condensed vapor is measured, andthe flow rate to the receiver is controlled to ensure that all of thevapor has been condensed in the heat exchanger 265. The condensed liquid250 from the receiver 240 is sent for processing in a process unit, suchas a dehydrogenator (not shown). A portion 255 of the condensed liquid250 from the receiver 240 is returned to the distillation column 215.

In some embodiments, the present invention could be used to provide allof the heating for the process stream. In other embodiments, it might beused to provide only a portion of the heating, with the remainder of theheating being provided in another manner.

Where the overhead vapor stream being used to heat the process stream isthe overhead vapor stream from a depropanizer, the overhead vapor streamis propane typically at a temperature in the range of about 40° C. toabout 60° C. This overhead propane vapor stream is typically condensedat a temperature of about 50° C., with the temperature setpoint for thetemperature controller being about 49° C. when the receiver pressure isabout 1.7 MPa (g) (about 17 bar (g)).

Where the stream being heated is the feed to a depropanizer column, andthe feed is coming from storage, it is typically at a temperature ofabout −35° C. The feed can be preheated using the present invention to atemperature in the range of about 0° C. to about 54° C., or about 20° C.to about 54° C. The feed stream should not be heated too high because oftreater requirements.

When using other overhead vapor streams and process streams to beheated, one of ordinary skill in the art will be able to determine theappropriate amount of cooling and heating for the overhead stream andprocess stream.

While at least one exemplary embodiment has been presented in theforegoing detailed description of the invention, it should beappreciated that a vast number of variations exist. It should also beappreciated that the exemplary embodiment or exemplary embodiments areonly examples, and are not intended to limit the scope, applicability,or configuration of the invention in any way. Rather, the foregoingdetailed description will provide those skilled in the art with aconvenient road map for implementing an exemplary embodiment of theinvention. It being understood that various changes may be made in thefunction and arrangement of elements described in an exemplaryembodiment without departing from the scope of the invention as setforth in the appended claims.

What is claimed is:
 1. A process for condensing an overhead vapor streamfrom a distillation column comprising: dividing the overhead stream intotwo portions; introducing the first portion of the overhead stream intoa first heat exchanger to exchange heat with a process stream therebyincreasing a temperature of the process stream and reducing thetemperature of the first portion of the overhead stream to condense thefirst portion of the overhead stream; controlling a flow rate of thefirst portion of the condensed overhead stream from the first heatexchanger; introducing the first portion of the condensed overheadstream into a receiver; introducing the second portion of the overheadstream into a second heat exchanger to reduce a temperature of thesecond portion of the overhead stream to condense the second portion ofthe overhead stream; and introducing the second portion of the condensedoverhead stream into the receiver and combining it with the firstportion of the condensed overhead stream to form a condensed liquid. 2.The process of claim 1 wherein controlling the flow rate of the firstportion of the condensed stream comprises: measuring a temperature ofthe first portion of the condensed overhead stream; reducing the flowrate if the temperature of the first portion of the condensed overheadstream is above a predetermined temperature, wherein the predeterminedtemperature is below a dew point for the overhead vapor stream; andincreasing the flow rate if the temperature of the first portion of thecondensed overhead stream is below the predetermined temperature.
 3. Theprocess of claim 2 wherein the predetermined temperature is at leastabout 1° C. less than a bubble point of the first portion of theoverhead stream.
 4. The process of claim 1 further comprisingintroducing the condensed liquid from the receiver into adehydrogenation reactor.
 5. The process of claim 1 wherein the firstheat exchanger is positioned above the receiver.
 6. The process of claim1 wherein the second heat exchanger is positioned below the receiver. 7.The process of claim 1 wherein the overhead stream comprises at leastone C₂ to C₆ alkane.
 8. The process of claim 1 wherein the processstream is a feed to the distillation column.
 9. The process of claim 8wherein the overhead stream comprises propane and wherein thetemperature of the feed to the distillation column is increased to atleast 0° C.
 10. A separation process comprising: separating a productstream in a distillation column into a liquid bottoms stream and anoverhead vapor stream; dividing the overhead stream into two portions;introducing the first portion of the overhead stream into a first heatexchanger to exchange heat with a process stream thereby increasing atemperature of the process stream and reducing the temperature of thefirst portion of the overhead stream to condense the first portion ofthe overhead stream; controlling a flow rate of the first portion of thecondensed overhead stream from the first heat exchanger; introducing thefirst portion of the condensed overhead stream into a receiver;introducing the second portion of the overhead stream into a second heatexchanger to reduce a temperature of the second portion of the overheadstream to condense the second portion of the overhead stream; andintroducing the second portion of the condensed overhead stream into thereceiver and combining it with the first portion of the condensedoverhead stream to form a condensed liquid.
 11. The process of claim 10wherein controlling the flow rate of the first portion of the condensedstream comprises: measuring a temperature of the first portion of thecondensed overhead stream; reducing the flow rate if the temperature ofthe first portion of the condensed overhead stream is above apredetermined temperature, wherein the predetermined temperature isbelow a dew point for the overhead vapor stream; increasing the flowrate if the temperature of the first portion of the condensed overheadstream is below the predetermined temperature.
 12. The process of claim11 wherein the predetermined temperature is at least 1° C. less than thebubble point.
 13. The process of claim 10 further comprising introducingthe condensed liquid from the receiver into the dehydrogenation reactionzone.
 14. The process of claim 10 wherein the first heat exchanger ispositioned above the receiver.
 15. The process of claim 10 wherein thesecond heat exchanger is positioned below the receiver.
 16. The processof claim 10 wherein the overhead stream comprises at least one C₂ to C₆alkane.
 17. The process of claim 1 wherein the process stream is theproduct stream.
 18. The process of claim 17 wherein the overhead streamcomprises propane and wherein the temperature of the product stream isincreased to at least 0° C.